Это пример кода. Для наложения двух виджетов, изображения и метки используется оверлей:
#include <gtk/gtk.h>
int main(int argc, char *argv[]) {
GtkWidget *window;
GtkWidget *image;
GtkWidget *label;
GtkWidget *overlay;
gtk_init(&argc, &argv);
window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER);
gtk_window_set_title(GTK_WINDOW(window), "Sandbox");
image = gtk_image_new_from_file("image.png");
label = gtk_label_new("I've always been too lame\n\
To see what's before me\n\
And I know nothing sweeter than\n\
Champaign from last New Years\n\
Sweet music in my ears\n\
And a night full of no fears\n\
\n\
But if I had one wish fulfilled tonight\n\
I'd ask for the sun to never rise\n\
If God passed a mic to me to speak\n\
I'd say \"Stay in bed, world,\n\
Sleep in peace");
overlay = gtk_overlay_new ();
gtk_container_add(GTK_CONTAINER(window), overlay);
gtk_overlay_add_overlay(GTK_OVERLAY(overlay), image);
gtk_overlay_add_overlay(GTK_OVERLAY(overlay), label);
g_signal_connect(G_OBJECT(window), "destroy",
G_CALLBACK(gtk_main_quit), NULL);
gtk_widget_show_all(window);
gtk_main();
return 0;
}